Line array speakers exhibit a number of advantages compared to point source designs. These advantages may include lower harmonic distortion compared to a speaker using a single drive element of the same type, as well as low-frequency directivity control and the creation of an extended nearfield zone. The nearfield zone is the area in which directly radiated sound predominates over sound reflected from room/hall boundaries, with attendant improvements in the clarity of reproduced sounds.
The beneficial directivity and nearfield effects of an array result from comb filtering, which is a combination of constructive and destructive interference. The constructive interference on-axis results, in the nearfield zone, in sound louder than would be obtained from an otherwise comparable point source design, while destructive interference off-axis results in cancellation and lower sound intensity than from a point source design.
The inherent low-frequency directivity control of line arrays has led to the widespread adoption of array designs particularly in large-venue sound reinforcement applications such as concert halls and stadiums in which it is important for power efficiency and clarity to direct relatively low frequency sound energy (in the range of 40-500 Hz) toward the audience. One drawback of an untapered array is however that at high frequencies the primary output lobe narrows significantly, which reduces the consistency of frequency response at audience positions not located within this primary lobe.
Array tapering schemes that have been deployed have therefore typically focused on achieving so-called constant beamwidth, i.e. managing the polar response of the speaker by narrowing the response at low frequencies so as direct soW1d energy toward the audience and to reduce the amount of delayed reflected sound reaching the audience; and by widening the response at high frequencies so as to maintain better matching of polar response with the low frequency output, thereby maintaining a more balanced frequency response across the widest possible angle of coverage.
For a variety of reasons, these tapering schemes have not focused on mitigating the inherent disadvantages of array comb filtering in the nearfield at high frequencies. These reasons include: (a) that the audiences in large venues typically are not placed in the nearfield, where high frequency comb filtering disadvantages are most pronounced; (b) that high frequency sounds are significantly absorbed over typical large-venue listening distances; and (c) that the disadvantages of nearfield high frequency comb filtering are most apparent when the reproduction quality is carefully optimized to create a stereo image for a single listener or small number of listeners, which is not of concern in large-venue sound reinforcement applications.
The ear is very sensitive to the arrival time and phase of high frequency sounds; the brain uses the arrival time and phase of high frequency sounds in particular to interpret spatial information. In home and studio applications optimized for quality of stereo image, very accurate reproduction of high frequency sounds is critical for the realistic reproduction of acoustic cues such as information about the dimensions and properties of the recording venue, localization of performers within the venue, etc.
Designers of loudspeakers for homes and audio studios have long sought to exploit the advantages of arrays, including the clarity provided by the nearfield effect. In the home or studio environment the distance from listener to speaker is typically short, often does not vary, and the listener often does not expect wide coverage of a large listening area, thereby presenting a much simpler set of required optimizations than in a sound reinforcement application.
However in home and studio applications the disadvantages of untapered line arrays are readily apparent, even when the listener is positioned within the primary lobe of the speaker's output. Home and studio listeners have consistently noted problems with arrays and quasi-arrays (e.g. tall panel speakers, whether electrostatic, planar magnetic, or ribbon type) such as acoustic image “stretch” along the axis of the array causing acoustic images to be inappropriately diffuse and large, the presentation of acoustic images at positions higher or lower in the sound field (closer to the ceiling or floor) than is realistic, and reductions in the ability to localize sounds precisely in comparison to the best point-source speak.er designs. These and related disturbances in the midrange and treble may also affect the perceived quality of lower midrange and bass sounds.
These deleterious effects of line arrays may result from progressively increasing comb filtering distortion at high frequencies, in which path length differences between the listener and various active points on the array become progressively larger in wavelength and phase terms with increasing frequency. While array comb filtering produces beneficial effects at lower frequencies (greater effective sensitivity and directivity compared to a point source using the same driver), as the reproduced frequency increases the path length differences between different points on the array to the listener become increasingly large in relation to the wavelength reproduced. A large number of effectively separate arrival times thus result for high frequency sounds, with the phase shift compared to the first-arrival sound steadily increasing as the source of the sound moves toward the ends of the array, creating comb filtering distortion. Thus the higher the frequency and the longer the array, the greater the disadvantage an untapered line array has over the point source from the perspective of multiple arrival times and comb filtering distortion.
Consider a vertical line array with radiating height of 2.5 meters positioned such that the listener's ears are 4 meters from the array at a height of 1.25 meters above the floor. The path length for unreflected sound from the middle of the speaker to the listener's ear is 4.0 meters, while by the Pythagorean theorem, the path length from both the top and bottom of the array will be 4.19 meters. This gives a path length difference of 0.19 meters from either top or bottom of the array to the listener compared to the path length from the middle of the array. At 100 Hz, the wavelength of the generated sound is 3.44 meters. The difference in path length in wavelength terms is therefore approximately 6% of the wavelength (0.19 meters divided by 3.44 meters). However at 15 kHz, the wavelength is 0.023 meters, meaning that the path length difference for the top or bottom of the array versus the middle of the array to the listener is more than a factor of 8 greater than the reproduced wavelength.
The listener in this example will perceive a low-frequency impulse as a coherent sound, but a 15 kHz sound will arrive at multiple different times. In psychoacoustic terms the listener does not consciously perceive the different arrival times as temporal problems, but as other forms of distortion as noted above.
Listeners in fact commonly find that the disadvantages of line arrays compared to point sources are rendered less objectionable or disappear entirely as the listener moves farther away from the speaker—that is, as the path length differences between the ends of the array and the middle of the array are reduced. However, by moving farther away from the line array to the point that path length differences are minimized, the listener removes him- or herself from the nearfield zone in which direct sounds predominate over reflected sounds.
Existing array designs for home and studio use have rarely employed electrical tapering or beam steering schemes. When these schemes have been employed, they have generally been either frequency independent, or frequency dependent with the intended effect of constant beamwidth as in sound-reinforcement applications. In practice these schemes have not removed the disadvantages at high frequencies of line arrays described above.
It is observed that the transition from nearfield, which comprises⋅primarily direct sound, to far field, comprising primarily reflected sound, occurs at a distance D approximately equal to 1.5 multiplied by the frequency reproduced (expressed in kHz), in turn multiplied by the length in meters of the array, squared, i.e., D=−1.5*f*h2
This implies that for a given nearfield listening position a longer array is required at low frequencies to place the listener in the nearfield than is required at high frequencies. A shorter array may be adequate to maintain the listener in the nearfield at high frequencies. A shorter array also geometrically reduces path length differences between the ends of the array and a listener at a fixed point. It may be possible to taper an array in a manner that minimizes or eliminates high frequency comb filtering distortion while maintaining a listener at typical home and studio listening distances in the nearfield zone.
It is therefore desirable to provide an invention that optimizes the effective radiating length of a line array speaker with increasing frequency so as to provide a nearfield listener at typical home or audio studio listening distances with the maximum array length possible (thereby maximizing the proportion of directly-radiated sound via the nearfield effect, as well as minimizing harmonic distortion) while simultaneously minimizing audible distortions from high frequency comb filtering, that is, minimizing the listener's perception of sounds emitted with psychoacoustically significant path length differences compared to the shortest path length from the speaker to the listener. Such an invention may in practice reduce or eliminate the perception of “stretched” images, inappropriate image location, and other problems typical of conventional line array or long panel or ribbon radiators, and thereby reduce or eliminate the primary objection many listeners have to line array speaker systems, while maintaining the described advantages of a line array design.